Efficiency Losses? Panels out vs Inverter out Watts

[Ellis Redding]

So looking at mounting 14 KW of panels (32 445W => 41.2V x 10.8A).
Using Enphase Micro inverters,
or really, seemingly any inverter, with 98% reported efficiency,
yet somehow when I calculate the output that comes out of the combiner,
it looks like its going to be about 240v x 32 x 1.35 max continuous out Amps => 10.36 KW
10.36 Out / 14.23 In = 73%

What gives here?

(for those very familiar with enphase I'll have 4 strings of 8 as the max per 20A breaker is supposed to be 11 which would be something like 15 A)

 

[BentleyJ]

The rating on a solar panel is based on Standard Test Conditions (STC) performed in a laboratory type of setting. 1,000 Watts/sq. meter of simulated sunlight and 25 deg C. In practice even when the panels are new and not degraded, the 73% ACTUAL output you calculated is normal. Our 2.82 KW system that is 11 years old maxes out at about 2.0 to 2.1 KW with the sun directly overhead and clear air. Any other conditions its less.
There are a online solar calculators where you can enter your zip code, solar panel compass direction & tilt, STC array size (14.23KW) and it will return a very good estimate of what you can expect on a monthly and yearly basis.

 

[Ellis Redding]

The rating on a solar panel is based on Standard Test Conditions (STC) performed in a laboratory type of setting. 1,000 Watts/sq. meter of simulated sunlight and 25 deg C. In practice even when the panels are new and not degraded, the 73% ACTUAL output you calculated is normal. Our 2.82 KW system that is 11 years old maxes out at about 2.0 to 2.1 KW with the sun directly overhead and clear air. Any other conditions its less.
There are a online solar calculators where you can enter your zip code, solar panel compass direction & tilt, STC array size (14.23KW) and it will return a very good estimate of what you can expect on a monthly and yearly basis.

Thanks, that makes sense.
Ok, then if the challenge is with the panels themselves, are all of the inverters out there way over sized?

 

[timselectric]

Thanks, that makes sense.
Ok, then if the challenge is with the panels themselves, are all of the inverters out there way over sized?

I don't understand the question. You should size the inverter to your needs. With a little extra cushion for the unknown.

 

[fratermus]

f the challenge is with the panels themselves, are all of the inverters out there way over sized?

I don't know how microinverters are sized in grid-tie but that's a reasonable question.

It's a different beast of course, but in offgrid scenarios mppt charge controllers are often overpaneled ≥10% (aka undersized). This is done to match the actual harvest panels are expected to make in the real world, rather than their theoretical max output. In the rare situations full power (or overpower) is available the controller trims power to match its rating.

 

[BentleyJ]

Thanks, that makes sense.
Ok, then if the challenge is with the panels themselves, are all of the inverters out there way over sized?

Solar panels exhibit other interesting phenomenon that has to be taken into account. Certain types of Partially Cloudy conditions like after rain fall when the air is super clear and the clouds start to "break up", there can be momentary sunlight magnifying events where the solar panels do actually put out 100% power or even 105%. I've have observed readings of around 3,000W from a 2.8KW array on the Fronius Inverter display that may last for a few seconds. Also, the Open Circuit Voltage of a solar panel increases as the temperature goes down.
Bottom Line: Everything in a Solar System, wires, fuses, breakers, Inverters, disconnects, batteries have to be sized based on the Maximum values of voltage and current that could ever be expected to occur, if only momentarily, but your day to day operating conditions and output is always less. So in this regard, the components are NOT oversized.

 

[OffGridInTheCity]

I have 45panels @ 285w each = 12.825kw PV array. I typically produce 18,000kwh/year based on the Midnite Classic 150 metrics. I have AIMS 88% peak efficiency inverters and I'm able to get 15,000kwh/year 'out of them' per cheap meters from ebay.

15000kwh/18000kwh = 83.3% overall system efficiency. Those are my numbers +/- the accuracy of the measurements. I attribute most of the loss to the inverters - e.g. say 14-15% leaving 1.7-2.7% to 'other' such as charge/discharge losses.

The AIMS are more efficient the closer I run them to max ability. For example, if I run at <30% max capability, my overall efficiency goes as low as 78%. If I run near max capacity my efficiency goes as high as 85%. So there's this idea of efficiency based on % of use of the inverter in question.

This is 3000kwh/year I'm leaving on the table! One does have to fight Solar OCD but in retrospect I'd wish I'd understood in my 'gut' that I'd be leaving 3000kwh/year on the table with less efficient inverters.

 

[MichaelK]

Solar panels exhibit other interesting phenomenon that has to be taken into account. Certain types of Partially Cloudy conditions like after rain fall when the air is super clear and the clouds start to "break up", there can be momentary sunlight magnifying events where the solar panels do actually put out 100% power or even 105%.

I've actually seen that. I've heard it called "cloud lensing". In my particular situation, with wispy Cirrus type clouds, I measured 1300W out of 1000W array, or 130%.

I was so stunned I kept running back and forth between the controller, and the array outside to monitor the clouds. It lasted less than 5 minutes, and I've never seen it again since.

 

[TorC]

Some panels are apparently better than others, and latitude makes a difference as well. I have 2S2P 320W REC NPeak panels (1280W nameplate) feeding one SCC, and regularly see it report 1.5kW peak output. I'm at ~22.9°N and partly cloudy with direct sun through openings is normal conditions, both of which sound like they would tend to increase max potential output. Even if the air isn't that hot, full sun can be brutally hot here.

 

[Ellis Redding]

I've looked at these 2 options for gauging output.

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

Solar Irradiance Calculator

www.solarelectricityhandbook.com

 

[wattmatters]

What gives here?

A whole range of things but there simply isn't enough information in your OP to provide comment.

1. system design is a function of many things
2. the rated output of solar PV array is but one of many of those elements
3. it's pretty common (and often good practice) to have a peak PV output rating somewhat higher than the inverter's AC output.

This item is one of the considerations:

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

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